Function monitoring circuits are known. These known circuits, however, do not distinguish between load drop-outs and short circuits to earth of an assembly, do not have any fault storage capacity which means that there can only be on-line fault detection, handle fault detection using a control circuit, and require extensive software search routines for fault detection and detection of the fault path and type.
One of the known function monitoring circuits uses the drive pins for signalling the output potentials of the output stage. During this process, it is determined whether the output stage which contains a transistor has assumed the "high" or "low" output potential. As a result, it can be determined after a software comparison of the input and output signal whether a short circuit of the output stage to the supply voltage or to the positive potential or to earth is present.
In another known function monitoring circuit, the output potentials of the output stages are signalled via a serial interface.
Another function monitoring circuit which is known signals several output stages simultaneously. These output stages, which are signaled by the function monitoring circuit, short circuit to the supply voltage and the load drops out on one side, and the load short circuits to earth on the other side at one connection pin each.
In this circuit, it is not possible to distinguish between faults--load drop-out and short circuits to earth. To make it possible to allocate the fault to one output stage, an extensive software search routine is necessary.